1. Field of the Invention
The present invention relates to a pneumatic tire, and specifically to a pneumatic tire which has improved driving stability and uneven wear resistance while maintaining good drainage performance based on main grooves each having a wide groove width and extending in a tire circumferential direction.
2. Description of the Related Art
General measures for improving the drainage performance of a pneumatic tire include a technique of arranging straight main grooves each having a wide groove width respectively in the center area and the two shoulder areas of the tread in the tire circumferential direction. A pneumatic tire with such straight main grooves has improved drainage performance, but suffers from the following problems. First, the straight main grooves cause a large columnar resonance while the tire is being run, and accordingly worsens noise performance. Second, since the straight main grooves each having the wide groove width are arranged even in the respective shoulder areas, the rigidity of each shoulder area decreases, and the driving stability accordingly decreases.
For the purpose of solving these problems, Japanese patent application Kokai publication No. 2004-168142 proposes a pneumatic tire which has a tread pattern as shown in FIG. 4, and for which a rotational direction R is specified. In this tread pattern, three main grooves 42, 43, 43 each having a wide groove width are arranged in the center area of a tread 41. Among the three main grooves, the main groove 42 located on the tire equator CL is a straight main groove, and the other two main grooves 43 located at two sides of the straight main groove 42, respectively, are each a wave-shaped main groove whose groove wall curves in the form of a wave. Since the three main grooves 42, 43, 43 each having the wide groove width are arranged in the center area of the tread 41 in the above-described manner, the drainage performance is improved. In addition, since each of the right and left main grooves 43 is formed by connecting multiple arc-shaped grooves 43a in series, the two sidewalls of the main groove 43 each curve in the form of a wave in the tire circumferential direction, thereby preventing the columnar resonance.
In each shoulder area, diagonal grooves 45a each having a narrow groove width and the other diagonal grooves 45b are arranged alternately in the tire circumferential direction. The diagonal grooves 45a extend obliquely outward from the respective connecting portions each between two neighboring arc-shaped grooves 43a of each main groove 43 in a direction reverse to a tire rotational direction. The diagonal grooves 45b each extend in parallel with the diagonal grooves 45a from an intermediate portion of each of the shoulder areas to the corresponding shoulder end. In addition, narrow grooves 46 each having a narrow groove width intersect with the diagonal grooves 45a and 45b in the tire circumferential direction. As a result, each shoulder area has a block pattern in which multiple blocks are arranged. In this way, instead of arranging the main grooves 42, 43, 43 each having the wide groove width, the diagonal grooves 45a and 45b and the narrow groove 46 each having the narrower groove width than those of the main grooves 42, 43, 43 are arranged in each shoulder area. This arrangement secures the rigidity of the shoulder area, thereby improving the driving stability in cornering driving.
However, the above-described pneumatic tire has a limit to further improve the driving stability, since the pneumatic tire is provided with the block pattern in which the multiple blocks are arranged in each shoulder area. In addition, since the diagonal groove 45a constituting the block pattern curves so as to have a convex shape protruding toward the inner side of the tire, leading-edge corner portions E each formed between each of the diagonal grooves 45a and the main groove 43 has a sharp acute angle. This presents problems of decreasing the rigidity of the corner portion E locally, and thereby easily causing uneven wear of the tire.
Furthermore, the diagonal grooves 45a and 45b located in the shoulder areas each curve so as to have a convex shape protruding toward the inner side of the tire. This makes the leading edge of each of the diagonal grooves 45a and 45b coincides with the front edge of the tread contact patch at a higher rate. This presents another problem that the front edge of each block makes a large hitting sound when the front edge thereof lands on the road surface, that is, pattern noise is large.